Alliteration occurs when the initial sounds of a word, beginning either with a consonant or a vowel, are repeated in close succession.

Examples:
Athena and Apollo
Nate never knows
People who pen poetry

Note that the words only have to be close to one another: Alliteration that repeats and attempts to connect a number of words is little more than a tongue-twister.

The function of alliteration, like rhyme, might be to accentuate the beauty of language in a given context, or to unite words or concepts through a kind of repetition. Alliteration, like rhyme, can follow specific patterns. Sometimes the consonants aren't always the initial ones, but they are generally the stressed syllables. Alliteration is less common than rhyme, but because it is less common, it can call our attention to a word or line in a poem that might not have the same emphasis otherwise.